Clearance seals and piston for cryogenic refrigerator compressors

Abstract

A cermet piston 36 is positioned to reciprocate within a ceramic sleeve 58 of a cryogenic refrigerator 12. The sleeve 58 forms a very tight clearance seal 67 with the piston 36 yet is allowed to float within a large gap 66 between piston housing 35 and the sleeve. The gap 66 allows the sleeve 66 to compensate for misalignment or distortions of the piston housing 35. Elastic seals 68, 70 prevent gas blow-by between the working space 18 of the piston 36 and the piston's dead space 19. The piston is connected to a motor drive mechanism through connecting rod 44 and helium lubricated wrist pin 48.

I claim: 
     
       1. In a cryogenic refrigerator, an assembly comprising: a piston housing;   a sleeve positioned to float within said housing and having a flexible seal between the sleeve and housing to permit transverse movement relative to the housing, the sleeve comprising ceramic and being a substantial peak to peak pressure variation at one end of the sleeve and a relatively constant pressure at the other end of the sleeve; and   a piston comprising ceramic positioned within said sleeve for axial reciprocation.   
     
     
       2. The cryogenic refrigerator of claim 1 wherein the sleeve is retained in the piston housing by means of a snap ring. 
     
     
       3. The cryogenic refrigerator assembly of claim 1 wherein the piston housing is comprised of two pieces and the sleeve is trapped in position by the assembly of the two piece piston housing. 
     
     
       4. The cryogenic refrigerator of claim 1 wherein said piston positioned within said sleeve has an outer diameter within 0.005 inches of the inner diameter of the sleeve. 
     
     
       5. The cryogenic refrigerator of claim 1 wherein the ceramic material of the piston is cermet. 
     
     
       6. The cryogenic refrigerator of claim 1 wherein the piston is driven by a rotary drive motor. 
     
     
       7. In a cryogenic refrigerator, a compressor piston assembly comprising: a two piece housing with a seal between the two pieces of the housing;   a sleeve comprising ceramic, loosely retained in positioned by the assembly of the two pieces of the housing;   a piston comprising ceramic for axial reciprocation within said sleeve, wherein the piston forms with one of said pieces of the housing a substantially closed work space for the compression of gas and wherein the other piece of the housing is open to allow connection of drive means to the piston;   a clearance seal between said piston and said sleeve; and   an elastic seal to prevent gas leakage between said sleeve and housing from said work space.   
     
     
       8. A helium cryogenic refrigerator of the type comprising a reciprocating displacer within a refrigerator cylinder, the displacer being driven in a reciprocating motion by the pressure differential between helium gas in the refrigerator cylinder and gas in a gas spring volume, the refrigerator cylinder being in fluid communication with a compressor space in a compressor cylinder, the compressor space and the refrigerator cylinder defining a closed working volume of helium gas which is compressed and expanded by a piston in the compressor housing; the refrigerator further comprising a fluid seal between the ends of the compressor piston which is a low friction clearance seal between a hard, smooth wall of a floating sleeve loosely positioned within the compressor housing and a hard, smooth piston wall to permit transverse movement relative to the housing.   
     
     
       9. A refrigerator as claimed in claim 8 wherein the radial clearance between the sleeve and piston is less than 1×10 -4  inches. 
     
     
       10. A refrigerator as claimed in claim 8 wherein the sleeve comprises ceramic material. 
     
     
       11. A refrigerator as claimed in claim 10 wherein the piston is comprised of cermet. 
     
     
       12. The cryogenic refrigerator of claim 8 wherein a crank mechanism is connected to the piston for oscillatory movement by means of a ceramic wrist pin, said pin forming a sleeve bearing and being lubricated by helium gas.